Acoustically-resistant couples play a significant role in determining and adjusting the acoustic characteristics and performance of earphones, especially when a particular frequency response characteristic is required. This is especially the case in the design of earphones which feature electronic ambient noise-cancellation (ANC) technology, and specifically to those utilising “ear-bud” type thin rubber flanges that seal the outlet conduit of the earphone into the entrance of the listener's ear-canal. Such earphones are sometimes referred to as “in-ear” earphones, or “ear-bud type” earphones, and they are now widely used for portable communications and entertainment applications whilst the listener is travelling, including listening to music and, in conjunction with cellular telephone handsets, for hands-free calls and conversations.
Although the thin rubber ear-bud flanges might appear to effectively “seal” the earphone assembly into the listener's ear-canal, an earphone thus positioned and located does not provide an effective acoustic seal between the listener's ear canal and the ambient environment, because low-frequency sound vibrations can still pass through the rubber flanges themselves. In addition, as already mentioned, and as disclosed for example in U.S. Pat. No. 4,852,177, acoustically-resistant couples are often incorporated into acoustic coupling pathways that are provided in earphone structures so as to adjust the acoustic performance for a desired frequency response at the listener's ear, and such pathways allow external sound energy to be transmitted directly through the actual structure of the earphone and into the ear-canal. Acoustic coupling pathways are often implemented as small apertures, with acoustical resistance provided by an acoustically resistive mesh material overlying an aperture. Such pathways are usually situated to provide an acoustic connection either between the outer ambient air and the internal space situated at the front surface of an internal microspeaker (or in the space behind it), or between these two internal spaces themselves, or some combination thereof, and these pathways contribute to the complexity of the acoustic structure of the earphone.
The general structure of a prior-art ear-bud type earphone 10 is shown in FIG. 1, in which a microspeaker 12 is sealed into a central substrate 14, which, in turn, is sealed to both a front housing 16 and a rear housing 18. The front housing 16 includes an elongate outlet port comprising an inner opening 20 coupled to an in-ear extension piece 22 on to which a rubber ear-bud flange 24 is affixed, and the rear housing 18 often is formed with one or more rear vents, such as 26, linking the rear of the microspeaker 12 to the external ambient. It is convenient to refer to the volume of air in the front housing 16, lying between the front of the microspeaker 12 and the inner opening 20 of the outlet port, as the “front volume” 28, and to the volume of enclosed air lying in the rear housing 18 behind the microspeaker 12 as the “rear volume” 30. The rear housing 18 is also used to carry and locate the electrical flex connections to and from the microspeaker 12, though these are not shown, for reasons of clarity.
As already mentioned, it is usual to provide the earphone 10 with one or more vents or acoustically-resistant couples, such as that shown at 32, in order to modify the frequency response to provide, for example, high-quality sound reproduction. Such couples usually include acoustic resistors, formed by sealing a thin, acoustically resistant nylon mesh (or similar) over a small diameter (<1 mm), short length (<1 mm) aperture in the housing. This is often done by means of small, double-sided adhesive tape discs, as illustrated in FIG. 2, which shows an acoustic resistor 32, comprising a nylon mesh disc 34 mounted on to an adhesive disc 36 in which there is a central aperture 38 defining the active area of the acoustic resistor 32.
Typically, the disc 34/36 has an outer diameter of 3 mm, and a central aperture of 1 mm. It is beneficial to deploy such a resistance either between the front volume 28 and the ambient, as shown in FIG. 1, or between the front and rear volumes 28, 30. This expedient provides an additional benefit, in preventing a total hermetic seal of the earphone in the ear of the user, which could otherwise cause an unpleasant “blocked ear” feeling in use.
Further, the provision of a pathway between the ear-canal and the ambient (either directly or via the rear volume 30) allows air to escape from the ear-canal when the ear-bud 10 is inserted. This prevents damage to the microspeaker 12 as, without such a pathway, the air in the canal and front volume 28 would be momentarily compressed, and this could force the diaphragm of the microspeaker 12 beyond its mechanical limits, potentially buckling the diaphragm and causing permanent damage.
In practise, only one of these acoustic couples is required to avoid the above problems: either a front volume-to-ambient couple, or a front volume to rear volume couple (assuming that the rear volume itself is also vented). The present invention utilises an acoustic couple between the front volume and the rear volume.
When it is required to implement a front volume to rear volume acoustic couple, such as in the acoustic module design disclosed in GB-A-2,475,526, it is convenient to position the elements of the acoustic couple directly adjacent to the microspeaker 12. This is illustrated in FIGS. 3(a) and 3(b), in which features corresponding to those already described with reference to FIGS. 1 and 2 are identified by the same reference numbers. FIG. 3 shows only part of an earphone 40, comprising a front housing 16 and its contents, but it will be appreciated that a rear housing, such as that shown at 18 in FIG. 1, would be attached to the front housing 16 to form an enclosed unit defining a vented rear volume, such as that shown at 26, 30 in FIG. 1.
In FIGS. 3(a) and 3(b), the front housing 16 of an earphone shown in part at 40 includes an acoustic resistor 42 mounted over an aperture 44 formed in the substrate 14, beside the aperture provided for the microspeaker 12, thereby providing an acoustic leakage path, via the resistor 42, between the front volume 28 and the rear volume (not shown in FIG. 3) of air in the earphone. However, this layout increases the lateral dimensions of the earphone 40 significantly beyond those needed to accommodate the microspeaker 12, as is clear from the drawing. In addition to the area of the acoustic resistor 42, which may typically have a diameter of around 3 mm, it is necessary to allow for manufacturing clearances around the edges of the individual components, and consequently the overall lateral dimensions of an earphone such as 40 are considerably larger than those of an earphone such as that shown at 10 in FIG. 1.
It is one object of the present invention to provide an earphone which incorporates a front volume to rear volume acoustic couple, without requiring significant addition to the lateral dimensions of the earphone.